1. Field of the Invention
The present invention relates to an electro-optical system and a method of displaying images. More particularly, it relates to such a device and a method of displaying color images without undesirable displacement of constituent color components.
2. Description of the Prior Art
As a prior art electric display, the cathod ray tube (CRT) such as the Braun tube has been broadly known. The CRT deflects electron beams in desired directions in order to form images on the screen. Since this mechanism is simple and capable of clear images, the CRT has prevailed for 70 or more years after the invention as a most useful display.
In recent years, along with the development of visual softs, wider screens and higher definitions are increasingly required. The CRT is not so excellent in this regard because a high vacuum is necessary for scanning electron beams. Considering the endurance of the tube, the CRT has to be formed with a heavy weight and a large size. For example, in the case of a screen with its diagonal dimension of 30inches, the thickness of the glass wall and the weight of the CRT are increased respectively to 1 cm or thicker and 100 kg or heavier.
In order to solve this problem, the projection display has recently been proposed and utilized. The basic mechanism of the projection display is same as that of the movie projector which was invented by Thomas Edison at the end of 19th century. In the case of the movie projector, light rays are passed through an Ag salt film and expanded to form a large size image. On the other hand, in the case of the recently developed projection display, two-dimensional optical switches such as liquid crystal displays panel, light valve devices are utilized in place of the Ag salt film.
FIGS. 1 and 2 illustrate typical configurations of the transmission projection display and the reflection projection display respectively. The transmission projection display is provided with three liquid crystal panels 708 to corresponding to three primary colors, i.e. red (R), green (G) and blue (B) which are combined and expanded in one screen as shown in FIG. 14. The alignment of the three panels and the associated optical system must therefore be made with a high accuracy, e.g. of the order of one micrometer.
In such a projection display, light rays passing through the liquid crystal panels each having a diagonal dimension of 3 inches are projected on a 100 inches screen located 4 to 5 meters distant from the liquid crystal panels to form expanded images. A very high definition of the liquid crystal panels is therefore necessary in order to clear image without dimness.
The mechanism of forming images by the use of the projection display will be explained with reference to FIG. 1. Light rays are generated from a light source such as a metal halide lamp 701 and enters an optical system 702 in which the light rays are separated and directed onto three paths. The light rays in these paths are passed through filters 705, 708 and 707 and the liquid crystal panels 708, 709 and 710 to form images of red, green and blue and overlapped with each other by means of an optical device 713 in order to construct full color images. Numerals 703, 704, 711 and 712 designate mirrors for reflecting the light rays in the respective routes.
It is the most serious problem associated with the projection display system that a greater portion of the light energy is dissipated in the form of heat since the light rays utilized are usually white light rays which are passed through the filters to utilize only one component of the three primary colors. The projected images is therefore substantially dark and can not clearly be seen with usual indoor lighting so that when the display is used, the lighting must be made relatively dark. The use of a more powerful light source or the use of a wider band filter may increase the brightness of the projected images. A powerful light source, however, generates much heat which increases much power consumption and necessitates a particular cooling device. This is not economical. The use of a wider band filter, on the other hand, limits the color range available.
The range of qualities of color is substantially limited in the case of a liquid crystal display as compared with CRTs. FIG. 2 schematically shows the ranges of color qualities realized by means of color filters, CRTs and laser displays. The central asterisk denotes white. The greater the position departs from the white, the purer the color is. The most outer curve defines single color lights. The largest triangle defines the range of laser displays. The triangle of broken line defines the range of CRTs. The smallest triangle defines the range of displays utilizing color filters. As shown in the figure, the range of display utilizing color filters is substantially smaller than that of the CRTs and the laser displays. The colors formed by combining three primary colors are limited within the triangle having its apices of the three primary colors. In order to make more wide the range, the purity of each primary color must be improved. The purities of the primary colors in the case of color filter can be improved by limiting the band widths. The brightness of the light rays passing through the filters is, however, decreased as the band widths are made narrow. On the contrary, the range of CRTs is substantially wide as compared with the range of liquid crystal panels. This is because light emission of CRTs has sharp line spectra corresponding to optical transition of inner-shell electrons located at d orbitals and f orbitals caused by electron beams.
It may also improve the brightness of the projection display to utilize three color cold-cathode tubes as the light sources. The cold-cathode tubes are, however, not point light sources required by the projection display.
In order to solve this problem, the inventors have proposed to utilize lasers of three colors as the light sources. Laser light can be point source and monochromatic light. FIG. 3 illustrates a projection display system utilizing lasers. The system includes three beam expanders 809, 810 and 811 respectively for expanding the areas of cross sections of laser beams of three primary colors emitted from lasers 812, 813 and 814. The expanded laser beams of the three primary colors are passed through three liquid crystal panels 804, 805 and 806 in order to construct constituent images of the three colors respectively. The constituent images enter an optical system 803 in which these three images are overlapped to form a full color image. The full color image enters another expander 802 to project an expanded image on a screen 801.
The laser projection display is capable of forming a variety of colors as understood from the largest triangle in FIG. 2. In this case, the red light source, the green light source and the blue light source are respectively a He--Ne laser, a Nb:YAG laser (second harmonic waves) and an Ar+ laser. As apparent from the figure, the color varieties reproduced by the laser projection displays surpasses those realized by LCDs and even those by CRTs. The laser projection display is particularly excellent in displaying colors of the greenish group which is difficult for CRTs. The screen of the laser projection display is bright since there is substantially no absorption by filters. A cooling device is needed only for cooling the laser and the necessary power thereof is only of such an order as generally required for electric appliances or lower. Furthermore, in the case of typical solid lasers such as Nd:YAG lasers and typical gas lasers such as He--Ne lasers, Ar+ lasers, maintenance activity is facilitated or almost unnecessary.
It requires, however, intensive human efforts and highly accurate control for preparation, tuning-up and maintenance in order to form a clear full color screen by overlapping three color images without displacement. Particularly, since setting-up is influenced by moisture and temperature change, there are many problems for use in the temperate zone in which while most demand is expected therein, the variation in humidity and temperature is significant.
Particularly, whereas the monochromatic characteristic of laser beams is effectively utilized in the prior art laser projection display, no consideration is paid to other excellent characteristics such as parallel travel with little spread and facilitation of alignment of optical axes obtained therefrom. As apparent from FIG. 3, laser beams emitted from the lasers are expanded in advance and passed through the liquid crystal panels. Because of this, three laser beams having optical axes of about 3 inches, like the liquid crystal panels, have to run in the system, resulting in the most significant obstacle in the effort of designing smaller systems. Furthermore, since the three laser beams must be kept parallel, tuning-up and maintenance require human efforts even if it is somewhat lessened as compared to the case of usual projection displays. Particularly, it is significantly difficult to set up the system in order to obtain three laser beams in parallel after passing through the liquid crystal displays. In fact, it is hardly realized to construct 100 inches or wider full color screens.
Furthermore, whereas at least three liquid crystal panels are necessary in any cases, such a liquid crystal panel is expensive resulting in an increase in production cost. Still further expensive is the cost required to construct the optical system for combining three laser beams. The cost of the optical system tends to account for a greater part of the total production cost of the display system. In addition, the maintenance of the display system requires highly dexterity and therefore is very difficult not only for end users but also for retailers. Accordingly, it is required to reduce the number of optical switching panels and the complexity of turning-up.
It has been proposed for dispensing with expensive liquid crystal panels to utilize light bulbs. FIG. 4 illustrates an example of such a display system. The system utilizes a metal halide lamp 903 as a light source. Light rays emitted from the lamp 903 are reflected on a mirror 904 and form a parallel beam passing through a ultraviolet light filter 905 and an infrared light filter 906. The light beam is then partially reflected on a semi-transparent mirror 907 to form a first beam directed downward in the illustration and a transmitting beam passing through without reflection to the left direction. The transmitting beams is partially reflected by another semi-transparent mirror 909 to form a second beam directed downward and a third beam passing through without reflection. The semi-transparent mirror transmits 909 the blue component of the incident beam and reflect the red component of the beam. The first beam is then passed through a green filter 908 and reflected on a light bulb 911 associated with a display 914 such as a CRT to form a green optical image. The second beam is then reflected on a light bulb 912 associated with a display 915 to form a red optical image. The third beam is then passed through a blue filter 910 and reflected on a light bulb 913 associated with a display 916 to form a blue optical image. The three beams forming the red, blue and green images are passed through and reflected on the semi-transparent mirror 907 and expanded by means of a lense 902 to project a full color image on a screen 901.
Also in the case of the display system as illustrated in FIG. 4, highly accurate alignment is required for the optical elements associated with the display system. For example, the accuracy is of the order of 1 micrometer.
The diagonal size of the light bulbs is usually 3 to 10 inches. The images formed in the light bulbs are overlappingly projected onto the screen 901 about 4 to 5 meters distant therefrom as a full color image of 100 inches. In order not to make the color image rough or dim, the light bulbs are required to form highly definition images. Usual light bulbs are, however, formed only by laminating photoconductive thin films and electro-optical films made of such as a liquid crystal material in order to cut down the production cost. In the light bulb of this kind, photoelectrons generated by partial irradiation are dispersed, if the resistivity of the photoconductive material is not sufficiently high, resulting in formation of dim images. In order to avoid such a problem, the photoconductive is required to have such a high resistivity as the photosensitive drum of an electrostatic copier has. The product of resistance and capacitance per picture element, however, becomes too large to follow the motion of images.
Furthermore, the conductivity of such a photoconductive material has a substantially non-linear dependence on the strength of the incident light so that it is very difficult to realize finely variable densities. Particularly, since incident light onto each light bulb carries analogue optical information, it is difficult to grade the brightness or the density. Furthermore, a greater part of optical energy of the light emitted from the lamp 903 is dissipated so that the screen can not be so bright.